Lesson 26: Venus, a Hothouse World
In this lesson, we will take a look at Venus"s surface and
the apparent geological activity. We will also look at why its atmosphere is so
different from Earth"s atmosphere.
Venus' thick cloud cover makes it hard to see to the surface. However,
Venus shows features of volcanoes and tectonics similar to
Earth. Venus lacks erosion because it is too hot for any type of rain or snow.
Also Venus has virtually no wind or weather because of its slow rotation.
Venus lacks plate tectonics, which are
important for shaping most of Earth's major geological features such as
mountain ranges and ocean trenches. This is a mystery to scientists as Venus
has a thick, strong lithosphere. However, scientists have guessed that Venus'
lithosphere is stronger than the Earth's and thus resistant to breaking into
plates.
Internal heat on Venus should be similar to the Earth due to their similar sizes,
which should lead to similar levels of geological activity.
In fact,
there are
relatively few impact craters meaning the surface is geologically young.
Furthermore, Venus's clouds contain sulfuric acid, which comes from volcanic
outgassing. Over time, sulfur dioxide disappears from the air, so its presence
means outgassing had to have happened less than 100 million years ago.
However, there is little evidence of erosion on Venus because it is too hot for
rain and the planet rotates extremely slowly (once every 243 days), so there is
no wind or weather.
One mystery of Venus is an apparent lack of plate tectonics. On Earth, stresses
caused by convection currents in the mantle fractured our lithosphere into
tectonic plates. The movements of these tectonic plates are responsible for
shaping most of Earth's major geological features such as mountain ranges and
ocean trenches, and also means that different regions of the Earth have
different geological features and different ages.
On Venus, on the other hand, the entire planet looks to
have aged uniformly. Scientists hypothesize that Venus has a thicker and
stronger lithosphere than the Earth, so it has resisted fracturing caused by
mantle convection.
Venus is a lot closer to
the Sun, but the density of its clouds means its surface actually absorbs less
sunlight than Earth. If it weren't for the greenhouse effect caused by a
carbon-dioxide heavy atmosphere, it would actually be colder than Earth.
Why are the Earth's atmosphere and Venus's atmosphere so different?
Earth and Venus both experienced the same volcanic outgassing which released
massive amounts of water vapor and carbon dioxide into the air. Water vapor and
carbon dioxide, as you may recall, both function as greenhouse gases. The
difference is, our water vapor condensed into rain and formed oceans. Liquid
water breaks down carbon dioxide, so our carbon dioxide dissolved in the oceans
and formed carbonate rocks.
Why didn't Venus form oceans?
Venus's closer proximity to the Sun would have resulted in higher surface
temperatures. Higher temperatures mean the air can hold more water, and also
mean faster evaporation of water. This all results in a positive feedback loop
- more water in the atmosphere traps more heat, which in turn leads to even
more evaporation and more water vapor in the air. Without oceans, the carbon
dioxide also accumulates, and the end result is a runaway greenhouse effect.
Where did the water vapor in the atmosphere go?
The sun's ultraviolet rays break down water molecules in
the air and cause their hydrogen atoms to
escape into space. In the end, all of Venus's water disappeared into space and
it was left with only carbon dioxide.